Electroluminescence (EL) is a well-known technology for flat panel display applications. An EL display is a thin, solid-state device, which includes a phosphor layer and dielectric layer(s) sandwiched between two electrodes. Upon application of a voltage above a certain threshold value to the electrodes, the phosphor layer emits light. A specific type of EL device for display applications that has been commercially successful since the early 1980's is called alternating current (ac) thin film EL. It has the advantage of being stable, with respect to operating time, and can provide high contrast images since the phosphor layer, being a thin film, is transparent. High contrast is achieved since ambient light does not scatter off the phosphor layer as it would from a powder phosphor device. The details of ac thin film EL devices are discussed in Electroluminescent Displays, Y. A. Ono, World Scientific ISBN 981-02-1921-0 (1995).
FIGS. 1(a) and (b) show a cross section of a typical EL display device and a cross section of a single pixel. A transparent glass substrate is coated with transparent electrodes (Indium Tin Oxide (ITO) is commonly employed). A first insulating layer is formed on top of the ITO, and a phosphor layer follows. For example, in commercially available EL displays the EL phosphor layer is ZnS:Mn. A second insulating layer follows, and finally a rear electrode is applied to complete the structure. Aluminum (Al) is commonly employed.
In operation, ac voltages in the form of alternating positive and negative voltage pulses are applied between the ITO and Al electrodes generating high electric fields in the phosphor layer. Above a threshold voltage, on the order of ±185 volts, the phosphor layer emits a light pulse substantially synchronized with the leading edge of the voltage pulse. Below this critical voltage, the phosphor layer still experiences electric fields, but the electric field is not sufficient to generate light in the phosphor layer, and so the EL device is in its dark or off state.
The structure of FIG. 1(a) also shows that in an EL display device, a plurality of ITO and a plurality of Al electrodes are created in the form of orthogonal stripes. We shall refer to the ITO stripes as columns and the Al stripes as rows. An EL display therefore contains a light emitting phosphor layer, which may be caused to light up in a desired spatial pattern. This is achieved by applying suitable voltages to the various rows and columns.
The intersection of the areas of any one row and any one column as shown in FIG. 1(b) incorporating the EL materials structure constitutes an EL pixel. This is the smallest light emitting element that can be controlled in the EL display. If there are N rows and M columns, then there will exist a total of N×M pixels. An example of monochromatic EL display constitutes N=480 (rows) and M=640 (columns) resulting in 480×640=307,200 pixels. This is known as a VGA format display. A full color VGA display would require 3×640 columns, since each pixel comprises three sub-pixels corresponding to red, green, and blue color emission.
In order to form an image in a practical EL display, an economical method of applying voltages to the N rows and M columns is employed. This is known as the matrix multiplex drive method or passive matrix addressing. Each row and each column is connected to a switchable voltage source. Solid-state semiconductor driver devices are commercially available that constitute the switchable voltage sources.
Consider the diagram of FIG. 2. The rows and columns of an EL display are represented by horizontal and vertical lines. The intersections of these lines represent the pixels of the EL display. Each pixel may be uniquely distinguished by identifying the numbers assigned to the row and to the column to which it belongs.
In order to create an image on the EL device, a sequence of events takes place very quickly such that the human eye cannot perceive the sequence of events, but sees the outcome which is a desired spatial pattern of lit and dark pixels which forms the image.
A number of EL drive methods have been developed (See Ono pages 100–111) which include a field refresh drive method, a p-n symmetric drive method and a p-p symmetric drive method. For illustrative purposes, a simple drive scheme is now described. To start with, all row voltages are set to 0 V. Firstly, the M pixels in row 1 of the EL display are addressed as follows: The M columns are set to voltages by the column drivers. These column voltages are either +25 volts or −25 volts, say, for the purpose of illustration. The column drivers are represented by switches in FIG. 2. The pixels that are to be “on” are assigned +25 volts, and the pixels that are to be “off” are assigned −25 volts on their respective columns and the difference between the two is called the modulation voltage, in this case 50 volts. Once this has been done, a high voltage row pulse is applied to row 1 only. The pulse is negative 200 volts, say. The row drivers are represented as switches in FIG. 2. The effect of this is to cause the pixels whose columns are at −25 volts to remain dark since the pixel voltage is the difference between row and column voltage or −200−(−25)=−175 volts. This is below the threshold voltage for the EL device which is assumed to be ±185 volts for illustrative purposes. On'the other hand, those pixels whose columns are +25 volts will emit a light pulse since the pixel voltage is now −200−25=−225 volts, which exceeds the threshold voltage by −40 volts.
The voltage on row 1 now returns to zero and then a new set of voltages is applied to the M columns. These voltages are once again either +25 volts or −25 volts, however the choice is governed by the information to be supplied to the pixels in row 2 of the EL display. The pixels in row 2 that are to be lit must now be supplied with 25 volts and the pixels that are to be dark are supplied with −25 volts. Once these column voltages have been established, a −200 volt pulse is applied to row 2 only and the appropriate pixels in row 2 will be lit. This row voltage then returns to zero.
The same sequence of events as described for pixels of rows 1 and 2 now applies to the remaining rows until all N rows have received one −200 volt pulse in sequence and every lit pixel has been provided with −225 volts and every dark pixel has been provided with −175 volts. At this point, the addressing sequence is half completed. This is called one frame.
Next, the columns are set to +25 or −25 volts to re-address the pixels of row 1 of the EL display. However this time the pixels to be lit are set to −25 volts and the dark pixels are set to +25 volts. Once these column voltages are present, a +200 volt row pulse is applied to row 1. The lit pixels therefore achieve a pixel voltage of 200−(−25)=225 volts which exceeds the threshold voltage by 40 volts and the dark pixels achieve a pixel voltage of 200−25=175 volts which is below the threshold voltage of 185 volts. Once this row pulse returns to zero volts, the columns are set for row 2 and another +200 volt row pulse is applied to row 2. This is repeated until all N rows have received a +200 volt row pulse. This is one frame, and constitutes the second half of the addressing sequence. Now the entire sequence is complete and it begins again immediately to retain the perception by the viewer of a constant image on the EL display. The lit pixels thereby remain lit since the lit pixel voltage reaches +225 volts and −225 volts during two consecutive frames, and the dark pixels remain dark since the dark pixel voltages do not exceed +175 volts and −175 volts during two consecutive frames. In order to prevent the human eye from perceiving the individual addressing steps, approximately 60 frames per second or more must be achieved. At lower frame rates, flicker will become apparent, and also display brightness will suffer. This implies that not very much time is available to address any given row of pixels. For a VGA display, for example, with a frame rate of 60 per second, there are 16667 microseconds available per frame. Since there are 480 rows that are addressed once per frame, there are 16667÷480=34.7 microseconds available to address each row. The column electrodes must be given enough time to reach the desired ±25 volt levels and then the row electrode must reach the required ±200 volt level and return to 0V within the 34.7 microseconds available.
Therefore, as the number of rows on a display increases, and for higher frame rates, the time required to set these column and row voltages becomes a fundamental constraint in display design and performance. Referring to FIGS. 1(a) and 1(b), it is clear that the EL structure is inherently capacitive in nature, and that from a circuit viewpoint it is connected in series to an external voltage source by resistive elements comprising the column and row electrodes, and the internal resistance of the voltage source. This resistive-capacitive combination implies a characteristic time constant which limits the speed of movement of charges in the circuit which charge and discharge the capacitive EL structure. This limitation on speed of movement of charge increases the time required for the column electrodes to reach their operating voltage, which reduces the maximum refresh rate (and therefore brightness) available to address the panel. The problem is compounded as the size and resolution of the panel increases.
A second effect of multiplexing is that it causes undesirable power dissipation to exist in an EL display operation.
A simple parallel plate capacitor is illustrated in FIG. 3. The capacitance is calculated from the formula C=0A/d. Here 0 is a constant, namely 8.85×10−12 F/m and r is the relative dielectric constant of the medium between the plates. A is the area of the plates and d is the distance between plates. The capacitor of FIG. 3 is connected in series with a resistor in a circuit as shown in FIG. 4, which can be used to quantify how much power is dissipated. A voltage source Vm is connected to a capacitor of capacitance Ce by means of a resistor R. Consider Ce to represent the capacitance of one pixel of an EL display, Vm to be the modulation voltage which is less than the threshold voltage, and R to represent an effective circuit resistance determined by the EL driver and the resistance of the row and column electrodes of the EL display.
When a voltage Vm is applied to the circuit, current flows through the resistor R, thereby dissipating energy. This energy is given by ½CeVm2. Once the voltage across Ce reaches Vm, no further energy is dissipated, but energy ½CeVm2 is stored in the capacitor. This means that energy is dissipated during a frame, whenever pixel voltages are changing, causing the charge or discharge of pixel capacitances without generating any light output.
The power dissipation (Pmod) due to driving the columns of an EL display with a modulation voltage Vm is normally the dominant power consumption of the EL display in a ¼ VGA or higher resolution panel. Pmod is affected by the image being displayed since different images require different voltage sequences on the column electrodes. Also, in popular drive schemes as described in Ono, rows are allowed to “float” rather than being clamped at 0 volts when not being supplied with a positive or negative voltage; A “worst case” value of Pmod is calculated to determine the maximum power that can be dissipated. This power becomes, for example, Pmod=¼N f Cp Vm2 for the p-p symmetric drive method (Ono P110). Here, Cp=NMCe is the total EL display capacitance, f is the number of frames per second, N is the number of rows in the display, M is the number of columns, and Vm is the modulation voltage supplied by the column drivers.
On page 110, Ono shows the components of power that are dissipated in a typical VGA format monochromatic EL display. The results show that over 12 watts of power can be dissipated in a VGA EL display just charging and discharging column voltages. Since the overall power dissipation in the entire display is under 16 watts, it is clear that over 75% of the overall power is being used for charging and discharging column voltages in the example illustrated.
A further difficulty arises in addressing an EL display. A column voltage swing is accompanied by electric current flowing to the addressed pixels. Since only microseconds of time are available between each row address, the charge must flow fast to charge up those pixels, for example, that are at the end of the columns remote from the driver connection, resulting in large electrical currents. This requires high current column drivers, which are expensive, and also requires that column electrodes must be sufficiently conductive to handle the large electrical currents. However, as column electrodes are made to be more conductive, by increasing thickness for example, it is increasingly difficult to maintain them optically transparent to allow the light to come out of the display. Highly conductive bus bars have been proposed to increase column conductivity, but these structures add cost and also reduce optical efficiency. Employing bus bars also further increases the peak current demands on the column drivers, thus further increasing their costs. The overall effect of the problems associated with passive matrix addressing, namely unproductive energy dissipation and limitations on refresh rate, is to limit the size and resolution of useful EL displays and to add cost to the electronic drivers.
Therefore it would be advantageous to provide an AC EL display device that reduces the aforementioned problems.